Optical communications is an increasingly important area for data transmission because it offers high transmission speeds and high bandwidth. The success of optical communications depends critically on the quality of optical fibers used in data transmission systems. Optical fibers must transfer optical data signals with high fidelity and low attenuation. The main contributions to signal attenuation are scattering losses and absorption. Scattering losses originate primarily from Rayleigh scattering and lead to signal loss through redirection of the optical signal from the fiber to the surroundings. Scattering losses depend both on the wavelength of the optical signal and the composition of the material used to make the optical fiber. The most practical material for optical fibers is silica (with and without dopants). Minimum losses due to Rayleigh scattering in silica occur in the wavelength interval from ˜1300 nm to ˜1600 nm and optical signal sources that operate in this wavelength interval are widely used in silica-based optical communications systems.
Absorption losses are caused by absorption of the optical signal by impurities in the composition of the optical fiber material. High purity materials are needed to minimize absorption losses. The most significant absorption in the desirable ˜1300 nm to ˜1600 nm wavelength interval of silica fibers is the “water peak”, which is a broad absorption band centered near 1380 nm that extends from ˜1350 nm to ˜1425 nm. The 1380 nm absorption originates from absorption by OH groups present within or on the surface of silica fibers. The OH groups are primarily bonded to silicon to from silanol (Si—OH) groups. In order to minimize absorption loss in silica fibers, it is necessary to minimize the concentration of silanol groups in the fiber.
Optical fibers are made by drawing fibers from a preform. The preform is consolidated silica glass that includes a series of concentric regions of silica glass that differ in the level or type of dopant. Control of the spatial distribution, concentration, and/or type of dopant in the fiber preform creates regions that differ in refractive index. The differences in refractive index are manifest in fibers drawn from the preform and define the different functional regions of an optical fiber (e.g. core vs. cladding, low index depressions, tailored index profiles). The conventional process for making optical fiber preforms entails deposition of silica soot onto a silica cane. The cane is fully consolidated, becomes the central portion of the fiber preform, and has the composition desired for the core of the fiber ultimately drawn from the preform. The silica soot can be deposited as a single layer with a single composition or a series of layers that differ in composition, where the compositions of the one or more layers are designed to provide the index profile desired in the cladding region of the fiber ultimately drawn from the preform.
Silica soot is usually produced by flame hydrolysis of SiCl4. The water byproduct of the flame hydrolysis reaction leads to high concentrations of OH in the silica soot as well as to high concentrations of OH groups at the surface and in the near-surface region of the cane. To reduce the concentration of OH groups, a dehydration step is performed after soot deposition. In the dehydration step, the soot and cane are exposed to a dehydration agent (e.g. Cl2) that acts to remove OH. The high porosity of the as-deposited soot facilitates removal of OH from the soot layer in the dehydration step. The densified nature of the cane, however, inhibits penetration of the cane by the dehydration agent and significant amounts of OH can remain in the cane portion of the preform. The presence of OH in the preform leads to incorporation of a high concentration of OH in fibers drawn from the preform and to correspondingly high attenuation losses for optical signals at or near 1380 nm.
In addition to the water byproduct of flame hydrolysis, OH can also be incorporated into the fiber preform through reactions of non-bridging oxygens with hydrogen. Non-bridging oxygens are coordinatively unsaturated and can be passivated with hydrogen. The passivation process forms silanol groups and provides OH groups that contribute to signal attenuation through absorption. Fiber preforms often contain a high concentration of non-bridging oxygen groups and the preforms, as well as fibers drawn from the preforms, often exhibit high sensitivity to hydrogen through hydrogen-induced conversion of non-bridging oxygens to silanol groups.
There is a need for fiber preforms that have reduced OH concentration and reduced sensitivity to hydrogen. Fibers drawn from such preforms are expected to have low attenuation losses in the vicinity of 1380 nm.